1. Field of the Invention
The invention relates to integrated circuits and, more particularly, to those comprising one or more physical fuses.
A physical fuse is a circuit element having an initial structure that can be destroyed irreversibly to give it a final structure with different properties. The initial structure is, for example, an insulator structure and the final structure is a conductive or resistive structure. Alternatively, the initial structure is conductive or resistive and the destruction of the fuse givers it an insulator structure.
The destruction is generally done either by a laser beam directed towards the fuse to make it melt or by an electrical blowing circuit capable of applying a current and/or voltage to the fuse that modifies its structure.
2. Description of the Prior Art
An example of a fuse is described in the patent application No. EP-A-0 408 419. This fuse is constituted by a very thin insulating layer between two conductors. It is blown by the application of a high voltage (on the order of 20 volts) between the conductors with a current of some milliamperes for some tens of milliseconds. The insulator layer then becomes resistive. The state of the fuse (whether intact or blown) is detected by a circuit that measures the current flowing through the fuse. This circuit is used to control other elements of the integrated circuit in which the fuse is inserted. The fuse is used, for example to irreversibly block the access to a non-volatile memory zone in the integrated circuit.
This type of fuse is frequently encountered in chip cards. Although the invention is not limited to fuses for chip card circuits, chip cards illustrate a problem that may be encountered with respect to fuses: these cards are thin and can therefore get twisted, giving rise to severe mechanical strains in the integrated circuit chip. In the case of the fuse indicated here above, the strains may lead to the destruction of the resistive connection set up during the blowing of the fuse. The fuse may thus return to a state close to its intact state, even though it had been blown in a way that was supposed to be irreversible. In other fuse structures, the strains may give rise to defects of the same type, namely defects by which a fuse returns to a state where it is equivalent to an intact fuse or by which an intact fuse behaves like a blown fuse.
This situation is not acceptable even if the probability of its occurring is low.
To improve the reliability of integrated circuits with physical fuses, the invention proposes the association, with the fuse, of an electrically programmable non-volatile memory cell, that is programmed at the same time as the fuse is blown. The state of the fuse is confirmed by the state of the memory. In particular, the blown state of the fuse may be confirmed by the programmed state of the memory in the applications where it is necessary to ensure that the circuit can continue to preserve the functions defined by the blown state of the fuse. If the blown structure subsequently takes on the characteristics of the intact structure in varying degrees, then the memory cell will take the place of the defective fuse.